Field of the Invention
The present invention relates to an image reading apparatus configured to read an image, for example, a scanner.
Description of the Related Art
Copying machines and multi-function printers include an image reading apparatus configured to read an image from an original. Well-known methods of reading using an image reading apparatus include a platen reading method in which an image is read by moving a reading unit over an original put on a platen and a flow reading method in which an image is read from an original that is being conveyed by an automatic document feeder mechanism. The reading unit includes a light emitter configured to irradiate an original and a light receiver configured to receive light that is reflected by the original. The image reading apparatus is configured to generate image data that represents the image on the original, based on the reflected light received by the light receiver.
A light emitting diode (LED) or a similar white light source is used as the light emitter. The light receiver includes as a light receiving element a photoelectric conversion element, for example. The photoelectric conversion element is built by, for example, forming an R (red) color filter, a G (green) color filter, and a B (blue) color filter through application on a light receiving surface in order to read color images. A charge-coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor is used for the photoelectric conversion element. The reading unit is configured to read an original by receiving, with the light receiver, diffused light of light that is emitted from the light emitter and irradiates the original. The image reading apparatus is configured to generate image data from luminance data that is obtained as a result of receiving light with the light receiver of the reading unit (hereinafter referred to as “read data”).
The following description takes as an example an image reading apparatus with a reading unit that uses a white LED for a light emitter and a CMOS sensor for a light receiver. The reading unit of this image reading apparatus is a linear sensor, and aligns a plurality of white LEDs and a plurality of CMOS sensors in a main scanning direction, which is orthogonal to a direction in which an original is conveyed, or orthogonal to a direction in which the reading unit moves. FIG. 19 is a schematic diagram of the alignment of CMOS sensors in the related art.
The CMOS sensors include three light receiving element lines arranged at given intervals in a sub-scanning direction and, in each light receiving element line, as many light receiving elements as necessary for the resolution of the CMOS sensors are aligned in a single line in the main scanning direction. Here, a gap of one-pixel height is put between one light receiving element line and another light receiving element line in the sub-scanning direction. The three light receiving element lines are a light receiving element line for reading an R image, a light receiving element line for reading a G image, and a light receiving element line for reading a B image. An R color filter, a G color filter, and a B color filter are therefore formed by application on light receiving surfaces of the three light receiving element lines, respectively. The color filters separate the colors of light diffused by an original, and the light receiving elements receive the light passing through the color filters.
The three light receiving element lines are arranged at given intervals in the sub-scanning direction, and thus the image reading apparatus does not read an R-color image, a G-color image, and a B-color image from the same point on the original at the same timing. The image reading apparatus therefore coordinates an R reading point, a G reading point, and a B reading point so that the three reading points are in the same place by advancing or delaying the reading timing of read data of one light receiving element line from the reading timing in another light receiving element line by a length of time corresponding to the gap in the sub-scanning direction. In the example of FIG. 19, the G light receiving element line and the B light receiving element line are arranged with a gap of two-pixel height and a gap of four-pixel height, respectively, from the R light receiving element line in the sub-scanning direction. The image reading apparatus accordingly uses R-read data as it is, reads G-read data at timing that is behind or ahead of the reading timing of the R-read data by a length of time corresponding to a two-pixel height in the sub-scanning direction, and reads B-read data at timing that is behind or ahead by a four-pixel height, to thereby execute processing of putting the R-color reading point, the G-color reading point, and the B-color reading point in the same place.
With this configuration, fluctuations in the moving speed of the reading unit in platen reading, or fluctuations in the original conveying speed in flow reading cause the time intervals between R reading, G reading, and B reading to deviate from given time intervals, and the R-read data, the G-read data, and the B-read data consequently shift in the sub-scanning direction. The moving speed of the reading unit in platen reading and the original conveying speed in flow reading are herein referred to as “scanning speed”. The shifts of the R-color read data, G-color read data, and the B-color read data in the sub-scanning direction are herein referred to as “sub-scanning color shifts”. When the sub-scanning color shifts occur, the outline of an achromatic image is colored, and a monochromatic line is misread as a color line in some cases. As a solution to sub-scanning color shifts, a method is proposed in Japanese Patent Application Laid-open No. Hei 11-112749, which involves detecting the scanning speed when an image is read and correcting the amounts of shifts of the read data in the sub-scanning direction.
There are other causes of sub-scanning color shifts than the scanning speed fluctuations described above, and there are many color shifts that are difficult to detect in advance, for example, a shift in the focal point of a CMOS sensor due to vibrations of the image reading apparatus, and a shift in the focal point due to the flapping of the original in flow reading.
The present invention has been made in view of the problem described above, and an object of the present invention is therefore to provide an image reading apparatus capable of correcting sub-scanning color shifts that are caused by various causes.